Recent studies have hypothesized that water can play a key role in melt generation beneath mid-ocean ridges. During passive upwelling, the presence of water induces melting at greater depth, producing a large region of deep, low-degree melts between the wet and dry solidi. These low-degree, water-rich melts dilute higher-degree, anhydrous melts formed above the dry solidus, resulting in a seemingly paradoxical relationship in which higher water contents correspond to lower mean extents of melting in submarine basalts. To test this model, we are carrying out a systematic geochemical study of melt inclusions and glasses using ion probe techniques. Specifically, we have measured volatile, major, and trace elements at two ultra-slow spreading ridge localities, the Southwest Indian Ridge and the Gakkel Ridge, and compare these data with models for ridge melting. In contrast to erupted basalts, which are thought to represent aggregated melts pooled over much the melting region, melt inclusions have the potential to preserve instantaneous fractional melts generated over a wide range of depths. Thus, melt inclusions provide a direct means of sampling the deep, hydrous, low-degree melts predicted by melting models.

Preliminary results show that melts represent low degree melts and that melt inclusions are more primitive than co-existing glasses. Furthermore, pressure estimates from CO2 and H2O contents indicate that deep crystallization is common at slow-spreading ridges. However, there is no clear relationship between depth and degree of differentiation, indicating that melts can pool at various depths within the upper mantle and crust prior to eruption.